1. Field of the Invention
This invention pertains to thermal retention devices for storing thermal energy and releasing the thermal energy over time. More particularly, this invention pertains to a thermal retention device which utilizes a phase change material as a medium for storing and releasing thermal energy.
2. Description of the Prior Art
The prior art includes so-called phase change materials for storing thermal energy. A phase change material is a material which includes a substance which changes phase (for example, from solid state to liquid state or from a liquid state to a gaseous state) upon the application of thermal energy to the material. The process of changing phase results in energy (referred to as latent heat) being stored in the material. When the phase change reverses, the latent heat is released.
Phase change materials can be provided with a wide variety of phase change temperatures (i.e., the temperature at which the phase change material changes phase). An example of a phase change material is described more fully in U.S. Pat. No. 5,211,949 which describes a hydrocarbon dispersed within a finely divided silica. The phase change material stays in a powder form above and below the melting point of the hydrocarbon. The hydrocarbon melting point can be selected to be any one of a number of different temperatures. The phase change material results in an effective storing of thermal energy with energy being released at about the melting point of the hydrocarbon.
Numerous devices containing phase change materials are known in the prior art. For example, seat cushions are formed which will include a foam pad impregnated with a phase change material. The seat cushion is contained within an envelope of plastic such as vinyl or the like. The entire cushion may be placed in a source of thermal energy (for example, a conventional microwave oven). Energy is applied to the phase change material and the cushion will retain an elevated temperature for a substantial period of time (commonly one to four hours).
Other means, in addition to phase change materials, are known for storing energy. For example, U.S. Pat. No. 5,300,105 to Owens dated Apr. 5, 1994 teaches a foam pad vacuum sealed in a plastic envelope. The envelope also contains a liquid which is heated by application of microwave energy. The '105 patent teaches several uses for the invention including therapeutic pads, toys and cushions.
While prior art microwave thermal retention devices are acceptable for many uses, there are certain applications where such devices have limitations. For example, the user of such a device must have access to a source of the energy. In the case of a microwave thermal retention device, the user must have access to a microwave oven to supply the necessary energy to cause the phase change material or other heat retention means to store the energy. For many users, access to a microwave oven is not practical. Also, for many users, there may be a substantial period of time between which the energy is applied and the device is to be used. An example of such an inconvenience is where a thermal retention device is to be used as a cushion for either outdoor activity or for stadium use at sporting events and the like. In such cases, there may be a substantial period of time between the application of energy to the device and the use of the device.